The present invention relates to a three dimensional imaging method and device, that is to say a method and device for obtaining three dimensional data of a target surface or other object, whether such data is displayed in three dimensional form or not. Indeed, the data may never be displayed as such, but may merely be used to control other equipment. Such a device is useful for supplying three dimensional data to other instruments. The data can be valuable in the science of robotics, where objects are required to be identified on the basis of their three dimensional shape and manipulated accordingly. Such data is also useful in monitoring the accuracy of the shape of a series of articles intended to be identical with each other.
One of the objectives of the present invention is to develop a low cost, compact, high-speed three dimensional imaging device to be used in automatic assembly. The ability to analyze the three dimensional data of a scene very quickly, i.e. in a fraction of a second, has many practical advantages, especially in an industrial environment. It reduces interference problems, as well as the results of mechanical, acoustic and electromagnetic vibrations. Moreover, moving objects can readily be surveyed and measured.
The output from such a three dimensional imaging device is similar to a television signal except that the amplitude of the signal is related to the geometric characteristics of the object. In contrast, ordinary two dimensional television cameras provide an output signal having an amplitude that is not geometrically related to the object but represents the surface reflectance properties of the object, combined with the ambient light conditions, its orientation and the intensity and spectral characteristics of such ambient light. The result is thus usually affected by the orientation of the object and the proximity of other objects. It is primarily for these reasons that the extraction of three dimensional features from a two dimensional image of a scene is difficult to realize.
There are many applications, such as in robotics, where a relatively low resolution, three dimensional image is sufficient to enable the equipment to discriminate between various known objects, as opposed to a detailed inspection task where it is necessary to make fine comparisons between the ideal shape and the actual manufactured part.
An objective of the present invention is to provide a system that is versatile in that it enables selection either of high resolution, i.e. sensitive discrimination, with some sacrifice in speed, or alternatively, extremely fast operation, e.g. down to 0.01 second for a typical image size of 128.times.128 pixels, for those situations where less fine resolution can be tolerated. Speeds of this order of magnitude are normally quite fast enough for most assembly tasks. Indeed they represent a substantial improvement over speeds hitherto available in three dimensional imaging devices.
Among the various techniques for obtaining three dimensional data that have been proposed in the past, the use of an active triangulation system employing a beam of radiation, e.g. laser light, that is projected onto an area of the surface to be examined, and a position sensitive detector for measuring deviations in the reflected beam, is the approach that is believed most likely to lead ultimately to achievement of the desirable criteria mentioned above. Systems employing such triangulation have, for example, been described by K. E. Morander in "THE OPTOCATOR. A HIGH PRECISION, NON-CONTACTING SYSTEM FOR DIMENSION AND SURFACE MEASUREMENT AND CONTROL," 5th International Conference on automated inspection and product control, 1980, pp. 393-396; and by T. Kanade and H. Asada in "NONCONTACT VISUAL THREE-DIMENSIONAL RANGING DEVICES," SPIE Proc. Vol. 283, 1981, pp. 48-53. The principles employed in these systems are explained in more detail below in relation to FIG. 1 of the accompanying drawings.
These systems have certain disadvantages, however, particularly in relation to requiring comparatively bulky apparatus and limiting the size of the field of view that can be obtained. They also exhibit shadow effect problems when used to obtain the profile of objects of certain shapes.